Battery chargers for nickel cadmium batteries have been used for many years. In past they were generally characterized in having relatively simple circuits for terminating the provision of charging current to a battery. In one such common circuit, as soon as a battery's voltage equaled or exceeded the voltage provided by a full wave bridge rectifier, the provision of charging current to the battery was terminated.
More sophisticated variants of this basic circuit were developed for nickel cadmium, and more recently nickel metal hydride, batteries. Numerous prior art disclosures have been made suggesting methods of charging, and determining the state of charge of, nickel cadmium and nickel metal hydride batteries, including:
______________________________________ Country Pat. No. Inventor/Applicant Issue Date ______________________________________ U.S.A. 3,944,904 Hase 1975 U.S.A. 4,016,473 Newman 1977 U.S.A. 4,237,411 Kothe et al. 1980 U.S.A. 4,554,500 Sokira 1985 U.S.A. 4,679,000 Clark 1987 U.S.A. 4,912,392 Faulkner 1990 U.S.A. 5,248,928 Gilmore 1993 ______________________________________
In some prior art circuits, the state of charge of a battery was determined by monitoring changes in battery voltage or temperature, or by measuring the average value of charge current provided to the battery. Yet other prior art circuits terminated battery charging when a certain predetermined amount of time had passed, or when a certain number of current pulses provided to the charging battery had been missed over a predetermined period of time.
Many prior art battery charging circuits provided constant current to the nickel cadmium or nickel metal hydride batteries being charged, and monitored battery voltage continuously to determine when to terminate charge. Nickel cadmium and nickel metal hydride batteries have relatively low internal impedances, usually on the order of about 50 milliohms. Thus, open-circuit voltages (OCVs) and closed circuit voltages (CCVs) measured in those types of batteries do not differ significantly. As such a nickel-cadmium or nickel metal hydride battery nears the end of the charge cycle, voltage typically rises, levels off, and finally decreases. This decrease in battery voltage during the charge cycle is often used to terminate the charge cycle.
Other prior art battery charging circuits provided constant current to the nickel cadmium or nickel metal hydride batteries being charged, and continuously monitored battery temperature to determine when to terminate charge. As nickel cadmium and nickel metal hydride batteries near the end of their charge cycle, battery temperature suddenly increases. This increase in battery temperature was often used to terminate the charge cycle.
Still other prior art charging circuits continuously measured the average amount of charging current provided to a battery, and terminated charging when the average amount of current fell below a certain minimum threshold value.
Most prior art circuits for charging or determining the state of charge of nickel cadmium or nickel metal hydride batteries were relatively inexpensive and easy to design owing to the ease with which relevant battery parameters (such as cell temperature or closed circuit voltage) could be measured.
Descriptions of prior art battery charging methods and corresponding circuits developed for nickel cadmium and nickel metal hydride cells are set forth in Sections 3.2 and 4.2 of the Gates Energy Products Application Manual (Preliminary) entitled "Sealed Rechargeable Batteries," the handbook entitled "Ni-MH Rechargeable Batteries" published in 1992 by Toshiba Battery Co., Ltd. in Japan, and in course materials written by Dr. El-Sayed Megahed et al., and published in 1994 by the College of Engineering at the University of Wisconsin-Madison entitled "Battery Charging and Control Methods."
None of the circuits or methods described in the foregoing references, however, found ready application to the problem of charging, and indicating the state of charge of, rechargeable alkaline manganese dioxide batteries. The behavior of rechargeable alkaline manganese dioxide cells differs significantly from that observed in rechargeable nickel cadmium or nickel metal hydride batteries. See, for example, the discussion in the OEM designer's guide entitled "The New Power--RENEWAL.RTM.Reusable Alkaline.TM.Batteries," published by Rayovac Corporation in 1994, the disclosure of which is hereby incorporated by reference in its entirety.
Rechargeable alkaline batteries have an internal impedance as high as 5 ohms, which is much higher than the internal impedance characteristic of nickel cadmium or nickel metal hydride batteries. The relatively high internal impedance of rechargeable alkaline batteries presented significant problems respecting the accurate measurement of battery voltage. As a result, the open-circuit voltage (OCV) of rechargeable alkaline batteries was typically measured between charging pulses for an accurate indication of the battery's state of charge. See, for example, U.S. Pat. No. 4,977,364 to Kordesch et al., the disclosure of which is hereby incorporated by reference in its entirety, wherein the technique of "IR-free" charging of rechargeable alkaline manganese dioxide cells is described. In nickel cadmium and nickel metal hydride, conversely, state of charge may be determined accurately using a closed circuit voltage (CCV) measurement obtained during charging.
In rechargeable alkaline batteries, battery voltage was discovered to increase rapidly during the start of the charge cycle, but to reach a plateau and remain essentially constant for the remainder of the charge cycle. The constant voltage portion of the charge cycle was found to correspond to a state of charge anywhere between about 50% and about 100% of full capacity. Thus, in a rechargeable alkaline cell, the state of charge of the cell could not be determined accurately by merely measuring its CCV or OCV. One of ordinary skill in the art will therefore appreciate readily that rechargeable alkaline batteries cannot be charged safely and reliably in prior art battery chargers designed for nickel cadmium or nickel metal hydride batteries.
One means of solving some of the unique problems presented by rechargeable alkaline batteries is described in U.S. Pat. No. 5,376,875, where Yee et al. employ logic circuitry to count charge and clock pulses. A state of full charge of a cell is indicated by measuring the time between current pulses, and determining that a full state of charge has been attained in the cell when the measured time exceeds a threshold value.